Cytochrome c Peroxidase (CcP) is a heme protein isolated from yeast that catalyzes the oxidation of two molecules of ferrocytochrome c to ferrocytochrome c concomitant with the reduction of hydrogen peroxide to water. The cation mutants we have on hand were designed to mimic APX and hence most likely bind potassium, because only one Asp is in the coordination sphere. We have introduced a Ca2+ binding site into CcP by introducing another carboxylate ligand and its crystal structure needs to be solved. Our interest stems from the fact that the other peroxidase structures we have solved, manganese, lignin and peanut peroxidase, all have Ca2+ at this site. We have succeeded in obtaining crystals of the Ca2+ binding mutant of CcP but their diffraction pattern extends to only around 2.5 using conventional rotating anode x-ray sources which do not allow us to refine the structure that would allow powerful insights into the mechanism of electron transfer between the engineered mutant and its redox partners. In addition obtaining accurately the atomic bond distances between the bound metal and the ligands coordinating them would require solving the structure at higher resolution.